In recent years, as mobile communication systems have been developed, portable telephones and portable information terminals have been rapidly spread. In this trend, manufacturers have been vying to develop smaller terminals with higher performance. The systems for portable telephones include both analog systems and digital systems, and the frequency bands used in those systems include various ranges, such as an 800 MHz to 1 GHz band and a 1.5 GHz to 2.0 GHz band.
Through the recent development of portable telephones, the portable terminals now exhibit high performance in a dual mode (such as a combination of an analog mode and a digital mode, or a combination of two digital modes: TDMA (time-division multiple access) and CDMA (code-division multiple access)) and in a dual band (a combination of a 800 MHz band and a 1.9 GHz band, or a combination of a 900 MHz band and a 1.8 GHz or a 1.5 GHz). Under the circumstances, parts used in those portable telephones have also been developed to achieve high performance. Meanwhile, there have been demands for smaller and less expensive portable telephones and parts.
In those portable terminals, duplexers are employed to conduct branching and generation of signals. A duplexer normally includes filters and an impedance matching circuit. In general, the filters include a band-pass filter, a band rejection filter, or a combination of these two filters. However, duplexers using surface acoustic wave filters have been developed recently, to achieve smaller terminals with higher performance.
FIGS. 1A and 1B illustrate such a duplexer. FIG. 1A is a block diagram illustrating the structure of the duplexer, and FIG. 1B shows the frequency characteristics of the duplexer. In the graph shown in FIG. 1B, the abscissa axis indicates the frequency (increasing toward the right), while the ordinate axis indicate the pass intensity (increasing upward). As shown in FIG. 1A, a duplexer 10 includes two filters 12 and 13, an impedance matching circuit 11 (hereinafter referred to simply as the matching circuit 11), a common terminal 14, and individual terminals 15 and 16. The filters 12 and 13 each includes a surface acoustic wave filter, and have different pass-band center frequencies F1 and F2 (F2>F1). The filter 12 is a filter for transmission, while the filter 13 is a filter for reception, for example. Hereinafter, the filters 12 and 13 may be referred to as the transmission filter 12 and the reception filter 13, respectively. In a duplexer for a 1.9 GHz band, for example, the difference in frequency between the frequencies F1 and F2 is approximately 100 MHz.
The matching circuit 11 is provided to prevent the filters 12 and 13 from degrading the filter characteristics of each other. The characteristic impedance of the filter 12 with respect to the common terminal 14 will be hereinafter represented by Z1, and the characteristic impedance of the filter 13 will be hereinafter represented by Z2. If the frequency of a signal inputted from the common terminal 14 is F1, the characteristic impedance Z1 of the filter 12 is equal to the characteristic impedance value of the common terminal 14, and the characteristic impedance Z2 of the filter 13 is infinity and has a reflection coefficient of 1, by virtue of the matching circuit 1. If the frequency of a signal is F2, the characteristic impedance Z1 of the filter 12 is infinity and has a reflection coefficient of 1, and the characteristic impedance Z2 of the filter 13 is equal to the characteristic impedance value of the common terminal 14.
Japanese Unexamined Patent Publication Nos. 6-310979, 10-126213, and 2001-267881 disclose duplexers using surface acoustic wave filters. Among the publications, Japanese Unexamined Patent Publication Nos. 6-310979 and 10-126213 suggest improvements on matching circuits. Japanese Unexamined Patent Publication No. 2001-267881 discloses a ladder type surface acoustic wave filter, a dual-mode type surface acoustic wave filter, and a high-frequency circuit for a wireless device equipped with a duplexer. Also, Japanese Unexamined Patent Publication No. 11-340772 discloses that a combination of a ladder type surface acoustic wave filter and a dual-mode type surface acoustic wave filter is employed as a dual band filter, instead of a duplexer.
As shown in FIG. 2, a duplexer that uses a ladder type surface acoustic wave filter for both transmission and reception is also known. Each resonator 20 in the ladder type surface acoustic wave filters is a single-port surface acoustic wave resonator that includes an interdigital transducer 21 and reflectors 22 and 23 sandwiching the interdigital transducer 21.
Although improvements on matching circuits are described in Japanese Unexamined Patent Publication Nos. 6-310979 and 10-126213, the transmission and reception filters of the duplexers are not explained in detail. Also, a ladder type surface acoustic wave filter and a dual-mode type surface acoustic wave filter are described in Japanese Unexamined Patent Publication No. 2001-267881, but disclosure has not been made as to the relationship between the power durability required in a duplexer and a surface acoustic wave filter, and the relationship between the transmission/reception band of a duplexer and a surface acoustic wave filter.
Furthermore, the surface acoustic wave filter disclosed in Japanese Unexamined Patent Publication No. 11-340772 is employed on condition that one of the two band center frequencies is two to four times as high as the other one. This disclosure does not concern a duplexer in which the characteristics rises (or falls) of a filter affect the characteristics falls (or rises) of the other filter.
In the duplexer shown in FIG. 2, the suppression of the low frequency band of the reception filter 13 that overlaps the pass band of the transmission filter 12 is not strong enough, and interference is caused in the transmission band, as indicated by the broken circle line in FIG. 3.